Update Purchase Order Header Flexfield using API

Summary:

We are looking to update PO header DFFs using an API.

Content (required):

I have been looking through Oracle docs on APIs that can help us and I've found the SOAP API to be most aligned with what we want to do. I found an example for creating a PO with flexfields. Combining that with the example on changing PO line DFF, I created an envelope, but it doesn't seem to be working. The API call returns a body that shows requestStatus is a success, but there are no actual changes to the PO. Looking at the below code snippet, can you tell what we might be missing?